The present invention relates, in general, to semiconductor devices, and more particularly, to a novel III-V field effect transistor.
In the past, the semiconductor industry has utilized a variety of methods to form ohmic contacts to III-V field effect transistors (FETs). One common method employs a lift-off technique to create a multi-layer metal contact. Lift-off techniques generally involve blanket metal depositions to a patterned photoresist layer. The multi-layer metal structure is formed through openings in the photoresist. The photoresist openings have sloping sidewalls to permit the photoresist to be dissolved in a solvent in order to remove portions of the metal. The sidewall slope must be very accurately controlled. This sidewall control requirement results in low process yields and corresponding high manufacturing costs.
In addition, these prior lift-off techniques have a minimum spacing between the transistor's gate, and the source and drain ohmic contacts that limits the minimum transistor size that can be obtained. Generally, a minimum gate to ohmic contact spacing of about one micron or greater is required to allow room for the photoresist layer. This minimum gate to ohmic contact spacing, and a minimum lift-off opening of about two microns limits the minimum transistor size to approximately seven to eight microns. Also, the minimum spacing prevents abutting ohmic contacts to adjacent structures thereby further limiting the transistor's minimum size.
The metals used for the multi-layer metal structure causes another problem. Gold typically is used as one metal of the multi-layer structure because gold lowers the structure's contact potential thereby facilitating ohmic contact formation. But, gold reduces the ohmic contact's thermal stability and renders the ohmic contact unsuitable for temperatures exceeding 350 degrees Celsius (350.degree. C.).
Accordingly, it is desirable to have a III-V FET that has ohmic contacts produced by etching instead of lift-off, that provides a gate to ohmic contact spacing of less than one micron, that abuts the ohmic contacts to adjacent structures, and that is devoid of gold.